The ability to accurately measure analytes in the blood, particularly glucose, is important in the management of diseases such as diabetes. Blood glucose levels must be maintained within a narrow range (about 3.5-6.5 mM). Glucose levels lower than this range (hypoglycemia) may lead to mental confusion, coma, or death. High glucose levels (hyperglycemia) cause excessive thirst and frequent urination. Sustained hyperglycemia has been linked to several of the complications of diabetes, such as kidney damage, neural damage, and blindness.
Blood glucose levels are maintained in many diabetics with routine injections of insulin. Unlike the normal functioning of the body's glucose control systems, injections of insulin incorporate no feedback mechanisms. Controlling glucose levels therefore requires continuous or frequent measurements of blood glucose concentration in order to determine the proper amount and frequency of insulin injections.
Conventional glucose measurement techniques require lancing of a convenient part of the body (normally a fingertip) with a lancet, milking the finger to produce a drop of blood at the impalement site, and depositing the drop of blood on a measurement device (such as an analysis strip). This lancing of the finger, at typical measurement frequencies of two to four times a day, is both painful and messy for the patient. The pain and inconvenience has additional and more serious implications of noncompliance, in that many patients will not maintain the recommended regimen of blood glucose measurement and thereby run the risk of improper glucose levels and consequent harmful effects.
In short, the inherent limitations of conventional blood glucose measurement techniques mean that patients either suffer this pain and inconvenience or neglect glucose monitoring and suffer the adverse physiological effects of improper glucose control. There is a clear need for a glucose measurement technique that minimizes or eliminates pain and inconvenience to the patient.
Devices have been described which use a pump to draw body fluid from the patient to a glucose detector or other analytical instrument. For example, U.S. Pat. No. 5,161,532 uses a pump to draw interstitial fluid from the skin to an integral glucose sensor. This system requires a pump capable of creating, suction at a level of about 200-400 mmHg. EP Publication 0 595 237 discloses an analytical device for measuring blood constituents such as glucose, which also requires a suction pump capable of creating, suction at a level of about 400 mmHg. Body fluid is also sampled through the skin with a suction pump in EP Publication 0 513 789.
In addition, devices have been described which use the local application of ultrasound to increase the permeability of the skin. Ultrasound is believed to disrupt the lipid layers between the keratinocytes in the stratum corneum, thereby increasing the permeability of the skin (Mitragotri et al, J. Pharm Sci 84:697-706, 1995). U.S. Pat. Nos. 4,767,402, 4,780,212, 5,115,805, and 5,421,816 discuss the application of frequency and/or modulation of ultrasound to increase the permeability of skin for the purposes of drug, delivery.
There remains a need for improved devices and methods for the application of a static pressure gradient to increase the effectiveness of ultrasound enhanced permeability, particularly with the objective of enabling, speed of extraction while minimizing tissue damage, better control of rates of extraction, and rapid and minimally invasive sampling of body analytes such as glucose.